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Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 3: Structure of Planetary Atmospheres Chris Parkes Rm 455 Kelvin Building

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Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 3: Structure of Planetary Atmospheres. Chris Parkes Rm 455 Kelvin Building. Revision. 2. Evolution of Earth’s Atmosphere. Habitable zone – liquid water The Oxygen cycle photosynthesis & Oxygen absorbtion in balance - PowerPoint PPT Presentation

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Page 1: Chris Parkes Rm 455 Kelvin Building

Planetary Atmospheres, the Environment and Life (ExCos2Y)

Topic 3: Structure of Planetary Atmospheres

Chris Parkes

Rm 455 Kelvin Building

Page 2: Chris Parkes Rm 455 Kelvin Building

2. Evolution of Earth’s Atmosphere

• Habitable zone – liquid water

• The Oxygen cycle– photosynthesis & Oxygen

absorbtion in balance

• Oxygen in atmosphere over time– 3 reservoir model: oxygen

produced by life, anaerobic/aerobic life

• Water Cycle• Carbon dioxide Cycle• Self-regulation and the

“Gaia” hypothesis

Atmosphere

Shallow Ocean

Deep Ocean

Reducing Oxygenating

volcanic gases

weathering

volcanic gases

photochemistry

photosynthesis

Revision

Page 3: Chris Parkes Rm 455 Kelvin Building

Structure of planetary atmospheres

Earth

Mars

Venus

Page 4: Chris Parkes Rm 455 Kelvin Building

Earth’s atmosphere

Thermosphere

Earth’s radius = 6350km (4000 miles)

90% of mass in ~ 10 Km

No definitive upper boundary of atmosphere … (10000km)

Page 5: Chris Parkes Rm 455 Kelvin Building

Distinct layers

Separated by boundary layers

Temperature profile different in each layer

Pressure decreases by factor of 10 every ~15km in altitude

Earth’s atmosphere

Page 6: Chris Parkes Rm 455 Kelvin Building

Layers of atmosphere• Energy comes from

sunlight

• Temperature structure due to interaction of sun’s rays with gases

Mesosphere

Page 7: Chris Parkes Rm 455 Kelvin Building

Troposphere:

• Lowest layer– 8-16 km in height (latitude & season)– ~90% mass of the atmosphere– Contains virtually all water vapour and aerosols– Capped by inversion layer – limits convection– region where convection occurs (i.e. weather, storms)– Most weather systems below tropopause– Little interaction with upper layers

• Temperature drops with altitude (climb a mountain)

– Visible light reaches surface and warms ground– Infrared light radiated upwards and warms

• Warmer closer to surfaceH

eigh

t (k

m)

00 300 600

50

100

150

TroposphereStratosphere

Mesosphere

Thermosphere

Page 8: Chris Parkes Rm 455 Kelvin Building

Stratosphere

• Second layer from Earth– Aeroplanes just reach into this– Extends from tropopause to ~50km – 99.7% of atmosphere below stratopause– Maximum ozone (O3) concentration at ~22km– “nacreous” clouds

• Temperature increases with height– O3 absorbs UV from sun– No convection, air cannot rise as higher air is hotter

• Air stratified, cooler to warmer layers, – hence ‘stratosphere’

• Stratsosphere occurs because O3 absorbs UV– Lack of oxygen on Mars/Venus means no stratosphere

Hei

ght

(km

)

00 300 600

50

100

150

TroposphereStratosphere

Mesosphere

Thermosphere

Page 9: Chris Parkes Rm 455 Kelvin Building

Ozone reduction / hole in Stratosphere

• No convection in stratosphere means pollutants remain

• Chloroflourocarboms (CFCs) pollutants destroy Ozone

• Ozone protects us from harmful UV – e.g. skin cancer in humans, plankton reduction

Montreal Protocol (1989) banned production

Stratosphere Ozone reduction of about 4% per decade since the late 1970s.Decrease in stratospheric ozone over Earth's polar regions

Largest Ozone hole recorded, South pole, September 2006NASA satellitesPurple – least Ozone

1992 2008

Page 10: Chris Parkes Rm 455 Kelvin Building

Nacreous clouds in Stratosphere

~15-25km above

- mostly over

polar region

during winter

- wavy clouds

showing winds

& waves in the

stratosphere

Page 11: Chris Parkes Rm 455 Kelvin Building

• 3rd Layer from ground– ~50 - 80km altitude– “Noctilucent” clouds

• Temperature decreases with height– Decreasing heating as far above Ozone layer– Top of mesosphere is coldest place in atmosphere

• Below water freezing throughout mesosphere

Mesosphere

- From freezing of moisture content in mesosphere- (Ice) cloud formation possible, structure due to convection - When troposphere is clear of cloud, cloud visible after sunset, hence ‘noctilucent’H

eigh

t (k

m)

00 300 600

50

100

150

TroposphereStratosphere

Mesosphere

Thermosphere

Page 12: Chris Parkes Rm 455 Kelvin Building

Thermosphere• 4th layer from earth

– ~80 - 400km– International Space Station in this

• Temperature increases with height – Temperature is “theoretical” due to low gas density – Cosmic rays (X-rays) ionise gas molecules

• First gases reached• Aurora

• Ionosphere - Band of ionised gas– ionised atoms/molecules and electrons– Reflects radio waves – long distance communication, not just line of sight

• Final Layer – 400 km onwards

• Low density gas– gradual boundary between atmosphere and space

• From here gases can sometimes reach escape velocity– leave atmosphere

• At 600 km all atoms are ionised

Exosphere

Hei

ght

(km

)

00 300 600

50

100

150

TroposphereStratosphere

Mesosphere

Thermosphere

Page 13: Chris Parkes Rm 455 Kelvin Building

Aurora: 100 – 120 km

Collision of high energy charged particle with gas in upper atmosphere

Excited gas atom fluoresces (colour depends on gases)

Motion of charged particles in Earth’s magnetic field

Page 14: Chris Parkes Rm 455 Kelvin Building

MagnetosphereRegion of Earth’s magnetic field

Protects Earth from ‘solar wind’ – deflecting charged particle radiation

needed for lifeVenus/Mars – lost elements of atmosphere due to solar wind

Van Allen Belts•3000 Km, 16000 Km•Charged particles trapped in Earth’smagnetic fieldInner – protonsOuter - electrons

Page 15: Chris Parkes Rm 455 Kelvin Building

Venus’ atmosphere

Dominated by CO2

•Venus closer to sum than Earth does not explain extreme temp. difference

CO2 greenhouse gas – boosts temperatures

Surface Temperature > an oven

Page 16: Chris Parkes Rm 455 Kelvin Building

Venus’ atmosphereWeather forecast: Hot, Cloudy, No wind

•Pressure: Gas density at surface 10% of water

• Weak Coriolis – no strong winds

• High pressure– efficient heat transport

– temperature same everywhere

• No axis tilt – no seasons

• Surface temperature drives strong convection

• Covered with Sulphuric acid

clouds - highly reflective

• Lack of magnetic field – solar wind stripped water

Page 17: Chris Parkes Rm 455 Kelvin Building

Thin atmosphereLow pressure & temperature

Major components (by volume)

95.3% carbon dioxide (CO2)

2.7% nitrogen (N2)

1.6% argon (Ar)

0.15% oxygen (O2)

0.03% water vapor (H2O)

Global dust storm

CO2 polar ice caps

Mars’ atmosphere

Page 18: Chris Parkes Rm 455 Kelvin Building

Water – is there life on Mars … ?

Present day surface temperature too cold, frozen as ice

Evidence of water on surface of Mars in the past

Page 19: Chris Parkes Rm 455 Kelvin Building

Mars’ atmosphereWeather Forecast: Cold, strong winds, dust storms

•Pressure < 1% Earth•Liquid water – evaporate or feeze•CO2 – but weak greenhouse effectas very little atmosphere

•Most lost to space•Frozen in polar caps

•Mars has seasons•Year twice as long•See future lectures

•Temperature difference on planet• strong winds •dust storms

Page 20: Chris Parkes Rm 455 Kelvin Building

Temperature Profiles: Venus, Earth, Mars

Page 21: Chris Parkes Rm 455 Kelvin Building

Why are Earth/Mars/Venus atmospheres so different ?

• Why does Earth have ocean’s but not Venus, Mars ?– Mars: lost due to solar wind, frozen in ice caps– Venus: too hot, escape to space– Earth: temperatures low enough to condense

• Why is there much less CO2 in Earth’s / Mars atmosphere than Venus?

– Similar amounts of outgassing on Earth and Venus

– Mars: 1) No magnetsosphere lost to solar wind

2) Frozen in polar dry-ice caps

– Earth: CO2 dissolved in Oceans forming carbonate rocks

Page 22: Chris Parkes Rm 455 Kelvin Building

Why are Earth/Mars/Venus atmospheres so different ?

• Why is only Earth’s atmosphere mainly Nitrogen and Oxygen ?– Earth: water, CO2 gone – hence Nitrogen

– Earth: Oxygen produced by life

• Why does only Earth have a stratosphere ?– Earth: has Oxygen, solar radiation forms Ozone– Ozone absorbs UV light – the stratosphere

Page 23: Chris Parkes Rm 455 Kelvin Building

Pressure variation with height:

Hydrostatic balance

(assume const. T)Pressure at a point

due to weight of air above

Using ideal gas law

ph = psea×e(-0.12h)

Page 24: Chris Parkes Rm 455 Kelvin Building

Temperature variation with height: Lapse rate

• In troposphere, temperature decreases with height at 6.5ºC/km• Parcel of air at A moves up “straight” line in Temp vs Height graph• At B it is unstable - warmer than

surroundings, so continues to rise.• At C it is stable - at same temperature

as surroundings, so stops.

Height IncreasesPressure decrease exponentially, Temperature decrease linearly

For ideal gas law – pV=T pressure decreases faster than Temperature Volume must increase to compensate

Temperature

Height

A

B

C

Typical real

temp. profile

Lapse rate

Page 25: Chris Parkes Rm 455 Kelvin Building

Cloud formation & Lapse Rate

• Now consider air containing water vapour at A

• Air cools, reaches dew point at B, and water condenses

• Forms clouds

• Dew point falls slowly with height (2º C/km)

Height

A

B

C

Typical real

temp. profile

Lapse rate

TemperatureDew point

Page 26: Chris Parkes Rm 455 Kelvin Building

Example exam questions

Q1. Name the distinct layers of earth’s atmosphere. How does the temperature vary with altitude within these layers?

Q2.Give evidence of possible past existence of water on Mars. Is there water on the surface of Mars currently? Why?

Q3. How does the presence of CO2 affect the surface temperature of Venus?

Next lecture – solar radiation, energy budget

Page 27: Chris Parkes Rm 455 Kelvin Building
Page 28: Chris Parkes Rm 455 Kelvin Building

Water – is there life on Mars … ?

Page 29: Chris Parkes Rm 455 Kelvin Building

How can water flow on the surface?images from Mars Global Surveyor

Water – is there life on Mars … ?